Irrelevant Elements, ACM/ICPC NEERC 2004, UVa1635

 这种题目最重要的是思路了清晰

#include <cstdio>
#include <cstring>
const int maxn = 100005;//sqrt(n)+1 is enough

int fac[100][2];
int fac_c[100];
int a[maxn];
void factor(int m) {
    int& num = fac[0][0];
    num = 0;
    for (int i = 2; i*i <= m; i++) {
        if(m%i == 0) {
            fac[++num][0] = i;
            fac[num][1] = 0;
            do {
                fac[num][1]++;
                m/= i;
            } while(m%i == 0);
        } 
    }
    if (m > 1) {
        fac[++num][0] = m;
        fac[num][1] = 1;
    }
}
bool check(int n, int j) {
    int num = fac[0][0];
    int a = n - j;
    int b = j;
    for (int i = 1; i <= num; i++) {
        //为了提高效率，只用检验m的分解式中的素因数
        int p = fac[i][0];
        int& q = fac_c[i];
        for (; a%p == 0; a/=p, q++);
        for (; b%p == 0; b/=p, q--);
    }
    for (int i = 1; i <= num; i++) {
        if (fac[i][1] > fac_c[i]) {
            return false;
        }
    }
    return true;
}
int main(void) {
    int n, m;
    for (; scanf("%d%d", &n, &m)==2;printf("\n")) {
        int cnt = 0;
        factor(m);
        memset(fac_c, 0, sizeof(fac_c));
        for (int i = 1; i < n; i++) {
            if (check(n, i)) {
                a[cnt++] = i+1;
            }
        }
        printf("%d\n", cnt);
        for (int i = 0; i < cnt; i++) {
            printf("%s%d",i==0?"":" ", a[i]);
        }
    }
    return 0;
}